1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for purifying an exhaust gas discharged from an internal combustion engine such as automobile engine, more specifically, the present invention relates to a storage-reduction type NOxpurifying catalyst in which the sulfur poisoning of the NOx storing material is remarkably reduced.
2. Description of the Related Art
The exhaust gas discharged from an internal combustion engine, such as automobile engine, can be purified by a three-way catalyst which promotes burning of carbon monoxide (CO), burning of hydrocarbon (HC) and reduction of nitrogen oxide (NOx).
This three-way catalyst is generally constituted by forming a coat layer of a catalyst support such as γ-alumina on a monolith substrate and loading a catalyst component such as platinum (Pt), palladium (Pd) and rhodium (Rh) on the coat layer.
The three-way catalyst can efficiently perform the above-described purification when the exhaust gas has a composition in a specific range. Therefore, the air-fuel ratio in the internal combustion engine is strictly limited so as to give an exhaust gas composition in a specific narrow range. However, in practice, the exhaust gas composition undergoes a certain degree of fluctuation, due to the time lag in the control system or the like, and sometimes deviates from the specific narrow range.
Here, a cerium-zirconium composite oxide can provide an oxygen storage capacity (OSC) to absorb oxygen in an oxidative atmosphere containing O2 in a relatively large amount and releasing oxygen in a reducing atmosphere containing CO and HC in a relatively large amount, because the Ce atom contained therein can cause a change in the valence number.
Accordingly, when a cerium-zirconium composite oxide is contained in the exhaust gas purifying catalyst, the exhaust gas composition in the vicinity of the catalyst component fluctuates less and the above-described exhaust gas purification can proceed more efficiently.
In addition, considerable heat is generated accompanying the oxygen absorption of the cerium-zirconium composite oxide and this heat can be utilized for warming of the catalyst at engine start-up and can expedite the starting of the catalytic activity. By virtue of these properties, the cerium-zirconium composite oxide is used as a co-catalyst for the exhaust gas purifying catalyst.
As for the related art describing such a cerium-zirconium composite oxide, Japanese Unexamined Patent Publication (Kokai) Nos. 10-194742 and 6-279027 and the like are known. In general, the cerium-zirconium composite oxide as the OSC material is considered to optimally have a cerium content as high as about 50 mol %.
On the other hand, for the purpose of advancing environmental protection, reduction in the generation amount of carbon dioxide (CO2) discharged from an internal combustion engine such as automobile engine and reduction in the generation amount of NOx are an important issue. In order to attain such reduction, a lean-burn engine has been developed for the purpose of improving the fuel consumption and a storage-reduction type NOx purifying catalyst obtained by adding a function of storing NOx in a lean atmosphere to a conventional three-way catalyst has been developed for the purpose of purifying the exhaust gas of the lean-burn engine. These have a certain success against the above-described problem.
In the lean-burn engine, the fuel is usually burned at an air-fuel ratio (A/F) in the lean (oxygen-excess) condition and temporarily and intermittently burned in the rich (fuel-excess) condition.
HC and CO in the exhaust gas are efficiently burned and removed in the lean condition by the action of catalyst in the oxidative atmosphere. On the other hand, NOx is absorbed by a storing material in the lean condition and this NOx is temporarily released in the rich condition and reduced and purified by the action of catalyst in the temporary reducing atmosphere.
For the NOx storing material of the storage-reduction type NOx purifying catalyst, an alkali metal and/or an alkaline earth metal are generally used. Such an NOx storing material is considered to change into a nitrate in the lean atmosphere and into a carbonate in the rich atmosphere and thereby perform the absorption and desorption of NOx.
The lean-burn internal combustion engine such as diesel engine is characterized in that the heat efficiency is high and, therefore, the generated amount of CO2 is small. On the other hand, this internal combustion engine is disadvantageous in that NOx is generated in a relatively large amount and moreover, particulates of harmful substances are generated. Therefore, it is an important problem to reduce both NOx and particulates.
To solve this problem, the present inventors have proposed, as described in Kokai No. 2001-271634, an exhaust gas purifying system (DPNR) where NOx and particulate both are simultaneously purified by combining the control of air-fuel ratio in the combustion system with a specific catalyst in the exhaust system.
In this system, a storage-reduction type NOxpurifying catalyst is similarly used as the exhaust gas purifying catalyst and the fuel is usually burned in the lean condition and temporarily and intermittently burned in the rich condition, whereby the particulates are burned and purified using the active oxygen generated from NOx storing material or the like accompanying the fluctuation in the exhaust gas composition and at the same time, NOx is reduced and purified.
In use of the storage-reduction type NOxpurifying catalyst, the lean atmosphere and the rich atmosphere must be swiftly changed over by the control of the air-fuel ratio and therefore, depending on the case, the cerium-zirconium composite oxide as the OSC material having an action of absorbing and releasing oxygen is used in a small amount only for the early warming of the catalyst at engine start-up.
However, the fuel for the internal combustion engine contains a trace of sulfur. This sulfur is oxidized in the internal combustion engine and is coverted into SOx which is an exhaust gas component. When this SOx comes into contact with the storage-reduction type NOx purifying catalyst, a part is oxidized even into SO3 by platinum or the like as a catalyst component and the NOx storing material such as alkali metal and/or alkaline earth metal forms a sulfate.
The sulfate is chemically rather stable, compared to a carbonate and a nitrate and the sulfuric acid radical is not easily desorbed from the NOx storing material even in the rich atmosphere at high temperature. As a result, the NOx absorbing and releasing action and/or the active oxygen producing action of the NOxstoring material are inhibited to incur aging reduction in the performance of the storage-reduction type NOxpurifying catalyst.
This reduction in the catalytic performance of the storage-reduction type NOx purifying catalyst by SOx, namely, sulfur poisoning is one of important problems to be solved in use of the storage-reduction type NOx purifying catalyst.